Fix RTD/GitHub configuration so PR RTD builds are reported on GitHub

[tsalo]

Summary

The build_docs CI step is done on CircleCI and takes ~20 minutes. CircleCI already has a bunch of jobs to run, so the docs-building step can block other jobs. Additionally, there have been times where the CircleCI build didn't match up with the RTD build.

Additional details

The main benefits are reduced testing time and better alignment between testing-generated docs and deployed (RTD) docs.

Next steps

1. Look into an RTD Action and see if I can build it in PRs.
2. Remove old CircleCI docs job.
3. Add new Action.

[tsalo]

It turns out ReadTheDocs can build pull requests automatically, and that option was even already selected in NiMARE's RTD settings (which of course makes the CircleCI-based docs job redundant). Those builds should send status updates back to GitHub in the PR, but RTD is not showing up as one of the authorized third party applications in Neurostuff's (or my personal) settings, which is apparently necessary for the status reports to be posted. I don't know how to fix that.

[tsalo]

I had to give RTD permissions in the NeuroStuff organization, which it didn't have. Basically I had to go through my account's authorized apps, then select RTD, then grant permissions for neurostuff.